Rain onslaught in 1998 surprised experts

Updated 1:24 pm, Wednesday, October 16, 2013

Major flood crests marked on the Seguin water treatment plant on the Guadalupe River show the record level of the 1998 flood.

Major flood crests marked on the Seguin water treatment plant on the Guadalupe River show the record level of the 1998 flood.

Photo: Forrest Mims III / For The Express-News

Rain onslaught in 1998 surprised experts

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Fifteen years ago Thursday, a vast volume of water vapor from a Pacific hurricane and the Gulf of Mexico converged over South Central Texas. The result was historic flooding so dramatic that the weather forecasters missed its timing and magnitude.

On Friday, Oct. 16, 1998, National Weather Service forecasters predicted rain would be triggered by the arrival of a front Saturday evening. At 1:35 a.m. Saturday morning, the NWS predicted, “Excessive rains are possible over the Central Texas Hill Country later this afternoon.” An hour later, the NWS forecasted that the arrival of a front likely would trigger heavy rain Saturday night or Sunday morning.

Apparently, the huge volume of supermoist air that funneled in over northern Bexar County didn't hear about the forecast for up to 4 inches of rain. Long before the front even arrived, intense rain began falling in the early morning hours as the storm rapidly expanded north toward Austin.

As a NWS report later observed, the event “developed explosively.”

San Antonio and Seguin received 16 inches of rain before the front arrived. New Braunfels received more than 20 inches. The storm focused much of its energy within a circle around and just below San Marcos, where it dumped from 28 to 30 inches.

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My place near Seguin received 18 inches, and the adjacent Geronimo Creek, one of the tributaries of the Guadalupe River, widened to 1,000 feet.

The result of the huge downpours was record flash flooding that took 31 lives and damaged or destroyed many homes and businesses, especially along the Guadalupe River from New Braunfels to Victoria. The flooding caused more than $750 million in damages.

Some of these details are from “Service Assessment: South Texas Floods, October 17-22, 1998,” a report by the National Weather Service.

“The reason for this intense development 12 hours before the front reached the area is not evident at this time,” the report acknowledges.

The report also discusses in detail the general accuracy of the flood forecasts after the intense rainfall arrived.

The 1998 flooding demonstrated that unprecedented weather events can slip by forecasters equipped with high-quality satellites images and the latest computer models.

The floods also showed that a hurricane can contribute to devastating rainfall hundreds of miles away. As for the loss of life, most fatalities occurred when drivers attempted to drive across low water crossings.

Central Texas is known for the severity of its flash floods. Those who reside in flood-prone areas will learn much by reading the NWS report. You can find this report and others that are available online by searching “Texas Floods 1998 NOAA.”

Forrest Mims III, an amateur scientist whose research has appeared in leading scientific journals, was named one of the “50 Best Brains in Science” by Discover Magazine. His science is featured at www.forrestmims.org. Email him at forrest.mims@ ieee.org.